All Free Physics MCQs with Answers

21. A pump raises 200 kg of water through 10 m in 20 s. Taking g as 10 m s-2, its output power is

• A. 100 W
• B. 1000 W
• C. 2000 W
• D. 20000 W

Explanation: The work done is mgh, which is 200 times 10 times 10, giving 20000 J. Dividing by the 20 s taken gives 1000 W, or 1 kW. Forgetting to divide by time leaves the answer at 20000, which is the energy in joules rather than the power in watts.

22. A ball is dropped from rest. Ignoring air resistance, as it falls its

• A. total mechanical energy increases
• B. potential energy converts into kinetic energy with the total unchanged
• C. kinetic energy converts into potential energy
• D. total mechanical energy decreases

Explanation: Gravity is a conservative force, so in the absence of air resistance the sum of kinetic and potential energy stays constant while potential energy is steadily converted into kinetic energy. With air resistance included the total mechanical energy would fall, the loss appearing as heat. Energy is never created, so the total cannot increase.

23. A machine takes in 500 J and delivers 350 J of useful output. Its efficiency is

• A. 30%
• B. 43%
• C. 70%
• D. 143%

Explanation: Efficiency is useful output divided by total input times 100, which is 350 over 500 times 100, giving 70%. The remaining 150 J is lost, mostly as heat through friction. An efficiency above 100% is impossible, so the last option can be discarded on principle alone.

24. The gravitational potential energy gained by a 2 kg mass lifted through 5 m, taking g as 10 m s-2, is

• A. 10 J
• B. 25 J
• C. 100 J
• D. 1000 J

Explanation: Potential energy gained is mgh, which is 2 times 10 times 5, giving 100 J. The route taken to lift the mass makes no difference, since gravity is a conservative force and only the change in height counts. The answer 10 J comes from omitting the height.

25. A body falling through a fluid reaches terminal velocity when

• A. the drag force becomes zero
• B. the upward drag and upthrust together balance the weight, so the net force is zero
• C. the body stops moving completely
• D. gravity stops acting on the body

Explanation: Drag increases with speed, so the body accelerates until the retarding forces grow to match its weight, after which the acceleration is zero and the speed stays constant. The body is still moving, and gravity still acts; what has vanished is the resultant force, not any individual one. This is why a raindrop reaches the ground at a survivable speed rather than accelerating for its whole fall.

26. According to Stokes's law, the drag force on a small sphere moving slowly through a viscous fluid is proportional to

• A. the square of the speed
• B. the speed
• C. the cube of the radius
• D. the inverse of the radius

Explanation: Stokes's law gives the drag as 6 pi eta r v, so it rises in direct proportion to both the radius and the speed, provided the flow stays laminar. At higher speeds the flow becomes turbulent and drag grows roughly with the square of the speed instead. The law is what allows viscosity to be measured by timing a falling ball.

27. The terminal velocity of a small sphere falling through a viscous liquid is proportional to

• A. the radius of the sphere
• B. the square of the radius of the sphere
• C. the inverse of the radius
• D. the viscosity of the liquid

Explanation: Equating the weight less upthrust to the Stokes drag gives a terminal velocity proportional to r squared and inversely proportional to viscosity, so a sphere of twice the radius falls four times as fast. This is why fine dust hangs in the air for hours while a pebble drops at once. Raising the viscosity lowers the terminal velocity rather than raising it.

28. Flow is described as laminar when

• A. the fluid particles move in irregular, mixing paths
• B. each fluid particle follows the same smooth path as the one before it
• C. the fluid is at rest
• D. the fluid is compressible

Explanation: In laminar or streamline flow the layers slide over one another without mixing, so the velocity at any fixed point stays constant in time and streamlines never cross. Above a critical speed the motion breaks up into eddies and becomes turbulent, which dissipates far more energy. The Reynolds number is what predicts which regime a given flow is in.

29. The equation of continuity for an incompressible fluid states that

• A. A1 v1 equals A2 v2, so the fluid speeds up where the pipe narrows
• B. A1 v1 equals A2 v2, so the fluid slows down where the pipe narrows
• C. the pressure is the same everywhere in the pipe
• D. the density changes along the pipe

Explanation: The same volume passes every cross section each second, so where the area falls the speed must rise in proportion. This is why a garden hose squirts further when a thumb covers part of the opening. The equation is simply the conservation of mass applied to a fluid of constant density.

30. Water flows through a pipe whose radius halves at a constriction. The speed of the water at the constriction becomes

• A. half
• B. double
• C. four times
• D. unchanged

Explanation: Area depends on the square of the radius, so halving the radius quarters the area, and by the continuity equation the speed must rise by a factor of four to carry the same volume per second. Answering double is the error of treating area as proportional to radius. This strong dependence is why small constrictions in a blood vessel raise flow speed so sharply.